Cryptographic systems typically operate by transforming data through a mathematical algorithm in conjunction with a secret key, referred to as a cryptographic key, to generate a digital signature (in public key systems) or message authentication code (MAC) (in symmetric key systems). Cryptographic systems typically include one or more processing devices utilized to perform the required operations along with one or more memory devices used to store the algorithms and secret key(s) utilized during the cryptographic operations. Cryptographic systems have many applications both for the secure transmission of information and for the authentication and verification of the source of information. One such application is the verification of payment of postage. The vast majority of the Posts around the world require prepayment for provided postal services. Prepayment, however, requires verifiable evidence of paid postage. One method for providing verifiable evidence of paid postage is the printing of an indicium, including a digital signature, that is generated by a cryptographic device, often referred to as a Postal Security Device (PSD), contained within a postage meter.
Verification of the digital signature proves that the indicium was generated by an authorized postage meter using the cryptographic key. The information printed in the indicium and access to a verifying key are sufficient for the detection of counterfeit indicia as long as the cryptographic key of the postage meter is confidential. As such, maintaining the security of such cryptographic keys in postage meters (or any other devices that include a cryptographic device) is critical to prevent the generation of counterfeit digital signatures or MACs. Accordingly, PSDs are typically provided with security to prevent such keys from being obtained. For example, the cryptographic keys are not permanently stored within the PSD, but instead are stored in volatile memory such that removal of power from the memory of the PSD will result in contents of the memory, e.g., the cryptographic keys, being lost due to the discharge of the memory cells. This requires that PSDs be provided with an uninterruptible source of power, typically in the form of an external battery, such that the contents of the memory will not be lost when the postage meter is not connected to a supply source (i.e., plugged into an electrical outlet). In addition, PSDs are provided with physical security, such as, for example, tamper barriers or the like, that will detect tampering with the device in an attempt to obtain the key. Such tampering can include, for example, physical penetration with probes or leads in attempts to obtain the cryptographic keys stored in the memory of the PSD. PSDs are required by the United States Postal Service to comply with Federal Information Processing Standards (FIPS) publication 140-2, level 3, issued by the National Institute of Standards and Technology (NIST), which requires that PSDs have a full envelope of physical tamper protection and detection which encloses all electrical nodes. Detection of a tamper activity results in the memory of the PSD being cleared, typically within nanoseconds of the tamper detection, thereby rendering any such tamper activity fruitless in obtaining the stored cryptographic keys.
There are known attacks aimed at defeating the security to obtain the data, e.g., cryptographic keys, stored therein. One such attack involves shorting the battery that supplies power to the memories in the PSD. This prevents the clearing from occurring, as the memory devices are no longer provided with power. To increase the decay time of the memory contents, it is also known to quickly drop the temperature of the PSD to very low levels, e.g., −20° C. or −30° C. Because of the cold temperature, the discharge of the memory cells will be significantly slower. The combination of the memory device being unable to actively clear and the slow discharge of the memory cells can allow sufficient time for an attacker to obtain the contents of the memory, e.g., the cryptographic keys, before the memory fully discharges and the data contained therein is lost.
Thus, using an attack similar to one described above, it may be possible to obtain the cryptographic keys stored within a cryptographic device, resulting in the ability to generate counterfeit digital signatures or MACs.